1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic apparatus.
2. Related Background Art
Nowadays, colorization of image information has been progressed, and there are needs for higher speed output, reduced costs, and more sharpness of hard copies accordingly. Thus, also in electrophotographic image forming apparatuses, various devices have been attempted to meet such needs.
For example, in order to meet the needs for high speed output, four image bearing members are provided so that, by effecting image formation (charging, exposure and development) independently for each color (referred to as xe2x80x9cfour-drum systemxe2x80x9d hereinafter), the image forming speed is increased by four times.
Further, by changing a fixing speed to cope with the kind of transfer material, high image quality has been achieved for various transfer materials. For example, when the image is formed on a thick sheet or an OHP sheet, generally, the fixing speed is decreased. When the fixing speed is decreased, by using a fixing convey belt, the fixing speed can be changed while keeping rotational speeds of the image bearing members (photosensitive members) constant.
However, since a distance between a transfer station and a fixing station cannot be decreased, in such a method, the entire image forming apparatus tends to be bulky. To avoid this, an attempt in which rotational speeds of the photosensitive members are changed has been proposed. By using such an attempt, the dimension of the entire image forming apparatus can be reduced and the cost can also be reduced.
However, in the above-mentioned conventional full-color image forming apparatus, although high speed output has been achieved to some extent, high speed output as in a monocolor image forming apparatus cannot be achieved. The reason is that, in the full-color image formation, the total amount of used toner is generally greater than that in the monocolor image formation, if the high speed output is attempted, there arises a problem that the fixing ability will be more formation.
The present invention aims to eliminate the above-mentioned conventional drawbacks, and an object of the present invention is to provide an image forming apparatus capable of achieving high speed image formation and high image quality.